Paving stones have been in use for thousands of years. Initially such stones were literally that, naturally occurring stones or rocks of various shapes and compositions. Even the earliest installers of such stones recognized that stones of regular shapes and sizes facilitated their installation and effectiveness in use. That doubtless led initially to care in selecting stones and placing them and later to preparatory shaping of stones to facilitate their installation and function. Eventually such regular shapes came to be embodied in man-made bricks and “stones,” including a variety of different shapes and compositions.
Notwithstanding thousands of years of human experience in paving a variety of different surfaces for a variety of different reasons and uses, the need continues for pavers that can better address certain requirements.
Among those requirements are the need to successfully transfer the load of vehicles that use the paving to the subgrade without damaging the base course or its stability.
As is explained by the ICPI Tech SPEC Number 1 (Glossary of Terms for Segmental Concrete Pavement), interlocking concrete pavement is a system of paving that consists of discrete, hand-sized paving units with either rectangular or dentated shapes manufactured from concrete. The units are placed in an interlocking pattern, compacted into coarse bedding sand, the joints filled with sand and compacted again to start interlock.
Interlocking results when frictional forces between paving units prevent them from rotating, or moving horizontally or vertically in relation to each other. Interlocking is also defined as the inability of a concrete paver to move independently of its neighbors. Friction forces enable load transfer among the paving units. There are three kinds of load transfer, vertical interlock, horizontal interlock and rotational interlock. Vertical interlock is achieved by shear transfer of loads to surrounding units through sand in the joints. Horizontal interlock is primarily achieved through the use of laying patterns that disperse forced from braking and accelerating vehicles. Rotational interlock is achieved by using pavers of sufficient thickness, placed closely together, and restraining them by a stationary edge such as a curb.
Interlocking is not, however, the only characteristic of concern. Many paver installations need to accommodate the flow of water through the paved surface and into underlying structures, installations of which pavers are sometimes referred to as permeable interlocking concrete pavement.
Likewise, it is important the pavers be attractive, easy to install manually or with mechanical systems, and that they be easy to manufacture economically, quickly and reliably without undue complexity in the manufacturing equipment and without undue difficulty associated with wear of the manufacturing equipment.